The toxicokinetic properties of liquid pharmaceutical compositions are unpredictable. It is important that an active pharmaceutical ingredient (API) is correctly formulated so that a safe, effective and controlled dosage is provided when the composition is delivered to a patient. This is particularly the case for inhaled compositions. Furthermore, liquid pharmaceutical compositions must have reliable long term stability to ensure that the dosage profile of the composition will be maintained after storage. This avoids the administration of incorrect dosages. Pharmaceutical compositions must also be formulated in such a way that administration is not unpleasant for a patient, for instance as regards taste and acidity.
RPL554 (9,10-Dimethoxy-2-(2,4,6-trimethylphenylimino)-3-(N-carbamoyl-2-aminoethyl)-3,4,6,7-tetrahydro-2H-pyrimido[6,1-a]isoquinolin-4-one) is a dual PDE3/PDE4 inhibitor and is described in WO 00/58308 (and is also sometimes referred to as RPL554). As a combined PDE3/PDE4 inhibitor, RPL554 has both anti-inflammatory and bronchodilatory activity and is useful in the treatment of respiratory disorders such as asthma and chronic obstructive pulmonary disease (COPD). The structure of RPL554 is shown below.

It is often preferable to administer RPL554 by inhalation because of its efficacy in the treatment of respiratory disorders. An effective method of administration is nebulisation. Franciosi et al. disclose a solution of RPL554 in a citrate-phosphate buffer (Efficacy and safety of RPL554, a dual PDE3 and PDE4 inhibitor, in healthy volunteers and in patients with asthma or chronic obstructive pulmonary disease: findings from four clinical trials, The Lancet: Respiratory Medicine November 2013; 1(9):714-27. DOI: 10.1016/S2213-2600(13)70187-5).